There is one form of hydraulic suspension system known as the dis-continuous system in which a pressure control valve regulates a controlled pressure by throttling pump flow to tank. A directional control valve (DCV) connects the controlled pressure to one chamber (e.g. chamber “A”) of a double effect actuator while connecting the other chamber of the actuator (e.g. chamber “B”) to tank, thus generating a load in one direction. To exert the load in the other direction, the directional control valve is switched position, connecting the controlled pressure to chamber “B” of the actuator and chamber “A” to tank. Thus, when the actuator is in the resistive mode (it travels in the opposite direction to the load it exerts) the flow out of the chamber which has its pressure regulated by the pressure control valve is re-circulated to the other chamber via the pressure control valve (PCV) and the directional control valve (DCV). As the pump has fixed displacement, and the PCV is throttling the flow to tank, power is still consumed by the system.
When this form of hydraulic suspension system is used with 2 actuators (generally one on the front and one on the rear), it is duplicated and uses a flow divider to share the pump flow between the front and rear systems.
A third dis-advantage is that the pressure control valve has a minimum controlled pressure relative to the tank pressure. Hence when the directional control valve is switched, it creates some discontinuity in the load exerted by the actuator. Discontinuity in flow will also occur with possible noise issues. To minimize those discontinuities, synchronization between the pressure control valve and the directional control valve is required, ensuring that the pressure control valve is regulating its minimum controlled pressure whilst switching the directional control valve.
In this form, when the system is required to change the controlled load, it regulates the pressure via the pressure control valve, changing the pressure in the “primary” circuit (hydraulic line from the pump outlet to the pressure control valve). The more compliance in the primary circuit, slower the system will be to increase the controlled pressure and the load which can be controlled by the system.
In another hydraulic suspension arrangement known as the continuous system, a PCV regulates the pump pressure, and pressure reducing valves (PRVs) regulate the actuator chamber pressures to change load without causing any flow/pressure discontinuity. All actuator chambers are controlled, and so load change between extension and compression (and vice versa) is continuous, avoiding the “dead-band” which inevitably occurs in dis-continuous systems.
The valve arrangement prevents re-circulation of actuator flow from one chamber of the actuator to the opposite chamber, because of the design of pressure reducing valves requiring them to control the downstream pressure in the actuator chamber by either supplying oil from the pump line or releasing oil to the tank.
Thus, when the actuator is in the resistive mode (it travels in the opposite direction to the load it exerts) the flow out of the chamber which is at the highest pressure is directed to tank whilst the flow which is required to fill in the chamber which is at the lowest pressure is taken from the pump which is at the highest pressure: energy is consumed within the system, showing the inefficiency of the valve arrangement.
In this arrangement, no flow divider valve is required. Hence pump flow is shared between a front and rear actuator as they require. The associated parasitic losses are also avoided.
In this arrangement the pressure controlled by the PCV is always greater or equal than the one which is regulated by the PRVs. In operating modes of the system where actuator load is changing, control strategies can be implemented to not decrease the PCV controlled pressure so that compliance of the “primary” circuit is not a limitation to the system response time. Further, this allows use of the “primary” circuit as an energy storage device.